In vehicles in recent years, headlamps have been spreading which incorporate discharge lamps, that is, high-intensity light sources that give a light field of vision. As for a discharge lamp ballast apparatus for lighting the headlamps incorporating the discharge lamps, miniaturization, efficiency improvement and cost reduction are always required. In addition, exclusion of mercury, which is an environmental load material and a constituent of the discharge lamps, has become a big problem.
Among the discharge lamp ballast apparatuses having these problems, many of the ballast apparatuses used for conventional discharge lamps (referred to as “conventional bulbs” from now on) that emit light with sealing mercury inside in addition to metal iodide (metal halide) such as sodium iodide and scandium iodide are used in such a manner as to set the lighting potential of the discharge lamp at a negative value to reduce devitrification. In contrast to the conventional bulbs, as for discharge lamps without using mercury (referred to as “Hg-free bulbs” from now on) whose discharging voltage during steady-state lighting is halved, ballast apparatuses used for them can halve the effect of the devitrification. Accordingly, the ballast apparatuses need not pay special attention to the lighting potential. Thus, to reduce the size and cost of the components, ballast apparatuses are advantageous which fire discharge lamps using plus potential that enables addition of battery power source voltage to a booster power supply for firing.
The Hg-free bulbs with the foregoing advantage, however, have to pass twice the current of the conventional bulbs during the steady-state lighting, thereby increasing the thickness of the electrodes. In addition, because of the difference in internal materials sealed, the internal gas pressure is higher, the thickness of a glass ball constituting a light-emitting bulb increases, and thermal capacity increases. Therefore, unless greater power is fed to them than to the conventional bulbs during the time from the breakdown at firing the discharge lamp to the start of the steady-state current, not enough heating is given. This increases the probability of ceasing the current on the way from the breakdown to the firing (tiring failure). In such a case, the discharge lamp ballast apparatus must start refiring immediately after the firing failure. In particular, it is necessary for the ballast apparatus for the Hg-free bulb to set the time allowed to repeat the refiring longer than that of the conventional bulb considering the firing failure due to a shortage of heating. It is considered to be a problem peculiar to the ballast apparatus for the Hg-free bulb.
As described above, a variety of problems arise as to the discharge lamp ballast apparatus. As examples of the conventional discharge lamp ballast apparatuses which try to deal with the problems, there are following conventional examples.
As a first conventional example, a circuit configuration is proposed which aims to miniaturize the discharge lamp ballast apparatus with a simple circuit configuration, and drives an H-bridge (H/B) type inverter to light the discharge lamp with negative potential. To operate the switching devices placed in negative potential, a level-shift circuit is provided (see Patent Document 1, for example).
As a second conventional example, an apparatus is proposed which aims at simplification and cost reduction of the circuit configuration of the discharge lamp ballast apparatus, replaces the level-shift circuit of the first conventional example with a bootstrap circuit, and lights the discharge lamp at plus potential (see Patent Document 2, for example).
The foregoing bootstrap circuit charges a capacitor for maintaining the ON state of a switching device placed at the higher potential side of the H-bridge-type inverter when the higher potential side switching device is in the OFF state and a lower potential side switching device connected in series directly thereunder in the bridge connection is in the ON state, and uses the power of the capacitor charged now as a power source for maintaining the ON state of the higher potential side switching device in the next half cycle. This makes it possible to turn on the higher potential side switching devices without continuous power supply from a low potential controlling power source, and to convert a DC (direct current) to an AC (alternating current).
Since the bootstrap circuit is simple and inexpensive, it is an effective driving means of the switching devices of the H-bridge-type inverter serving as an alternating current converting circuit that always alternates polarity.
As a third conventional example, a configuration is proposed which aims to drive the switching devices constituting the H-bridge-type inverter stably, and has a bootstrap circuit with nearly the same configuration as that of the second conventional example. The third conventional example, however, is characterized by using an auxiliary power source to secure a control power source that also serves as the driving power source of the H-bridge-type inverter even at the time when the power source voltage drops (see Patent Document 3, for example).
As a fourth conventional example, an apparatus is proposed which aims at miniaturizing the discharge lamp ballast apparatus, and has a bootstrap circuit in the same manner as the second conventional example or third conventional example. To enable the switching devices placed at the higher potential side to maintain the ON state for a long time, a power source circuit with higher potential than the potential of the switching devices is provided so that the high potential power source supplies a current continuously to capacitors serving as a power source for turning on the higher potential side switching devices (see Patent Document 4, for example).
As a fifth conventional example, a circuit configuration is proposed which aims to start the discharge lamp without fail. It differs from the first to fourth conventional examples in that it drives the H-bridge-type inverter using a transformer (see Patent Document 5, for example).
Although ordinary driving of the switching devices with a transformer cannot continue the ON state of the switching devices for a long time just as an ordinary bootstrap, the fifth conventional example is characterized by enabling them to continue the ON state for a long time by providing each of switching devices at the higher potential side and lower potential side, which pair at passing the current, with an insulated DC power source to supply current to each of them.
Patent Document 1: Japanese Patent Laid-Open No. 10-41083/1998
Patent Document 2: Japanese Patent Laid-Open No. 2000-166258.
Patent Document 3: Japanese Patent Laid-Open No. 10-321393/1998.
Patent Document 4: Japanese Patent Laid-Open No. 4-251576/1992.
Patent Document 5: Japanese Patent Laid-Open No 6-196285/1994.
The conventional discharge lamp ballast apparatuses are configured as described above. Thus, as for the first conventional example, the circuit configuration based on the level-shift circuit can operate the switching devices placed at the negative potential in a DC mode, and select an apply voltage polarity and time for firing the discharge lamp optionally. Accordingly, although it can facilitate firing the discharge lamp stably, it requires a complicated level-shift circuit. In addition, to provide a negative DC power source, it must generate all the output power via a DC/DC converter without adding the DC power of the power source. As a result, it entails the transformer and the switching devices with rating satisfying the output power, which presents a problem of limiting the miniaturization or cost reduction of the discharge lamp apparatus.
As for the second conventional example, the capacitors constituting the bootstrap circuit can maintain the ON state of the switching devices at the higher potential side only during a limited time period during which the capacitor has charged power. Therefore, as at the time of firing, when the ON time of the higher potential side switching devices must be longer than that at the steady-state lighting, it is necessary for the capacitors that operate as the power source to secure the power for a longer time. For example, if firing failure is repeated, the ON time sometimes has to be maintained for one second. Thus, as long as the capacitors with a limited size are used, the polarity of the applied voltage for firing the discharge lamp cannot be fixed for a desired time period (the foregoing one second, for example), which present a problem of making it difficult to fire the discharge lamp stably in any conditions.
In this case, although using capacitors with large capacitance can implement a long ON state, it entails an increase in space and cost for mounting the capacitors with large capacitance unnecessary at the time of steady-state lighting, which is unfavorable for the discharge lamp ballast apparatus for headlamps. In addition, extension of the operation time has correlation with the capacitance of the capacitors, and the selection of the capacitors in the limited space of the ballast apparatus has only narrow freedom (particularly when extending time).
As for the third conventional example, it has potentially the same problems as the second conventional example about the ON time. Thus, it has the same problem in that it is difficult to fire the discharge lamp stably.
As for the fourth conventional example, the high potential power source enables the higher potential side switching devices to maintain the ON state for a longer time, and makes it possible to select the voltage apply duration and the voltage polarity for firing the discharge lamp freely, thereby facilitating firing the discharge lamp stably. However, to enable the higher potential side switching devices to achieve the longer ON time, the fourth conventional example supplies the power to the switching devices on the right and left arms in the same manner. The circuits of the two arms, which operate alternately, have the same potential difference as the power source voltage of the H-bridge-type inverter. Accordingly, the capacitors operating at the low voltage side must be charged via a current limiting series resistor to prevent an overcurrent. This of course increases a loss due to the resistor, but also presents a problem of involving an increase of the space and hindering the miniaturization of the ballast apparatus because the voltage applied to the resistor is high and hence a resistor with a high withstanding voltage or resistors connected in series must be use.
As for the fifth conventional example, although it can construct the DC/AC inverter of the discharge lamp ballast apparatus by using the transformer, the transformer is an electronic component whose characteristics are affected by the size thereof. Accordingly, the transformer necessary for the discharge lamp ballast apparatus requires larger space and higher cost than the semiconductor level-shift circuit used in the first conventional example or the bootstrap circuit using the capacitors of the second conventional example, which implements the space-saving, inexpensive circuit configuration. Thus, the fifth conventional example has a problem of being unfavorable as a circuit configuration for the discharge lamp ballast apparatus for the headlamps.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a discharge lamp ballast apparatus capable of achieving the miniaturization and cost reduction to enable application to the headlamps of a vehicle, and capable of lighting the discharge lamp stably.